Late Eocene impact ejecta in Italy: Attempts to constrain the impactor composition from isotopic analyses of spinel-rich samples

Christian Koeberl, Alessandro Montanari, Toni Schulz, Jonas Tusch, Florence Mougel, Frederic Moynier

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Abstract

The late Eocene was marked by multiple impact events, possibly related to a comet or asteroid shower. Marine sediments worldwide contain evidence for at least two closely spaced impactoclastic layers. The upper layer might be correlated with the North American tektite-strewn field (with the 85-km-diameter Chesapeake Bay impact structure [USA] as its source crater), although this is debated, whereas the lower, microkrystite layer (with clinopyroxene [cpx]-bearing spherules) was most likely derived from the 100-km-diameter Popigai impact crater (Russia). The Eocene-Oligocene global stratotype section and point is located at Massignano, Italy, and below the boundary, in the late Eocene, at the 5.61 m level, shocked quartz and pancake-shaped smectite spherules that contain (Ni- and Cr-rich) magnesioferrite spinel crystals are found. These are associated with a positive Ir anomaly in deposits with the same age as the Popigai-derived cpx spherule layer. This layer is overlain by another Ir-rich layer, likely due to another large impact event. From a large amount of “pancake-bearing” rock, we isolated a few hundred milligrams of this spinel-rich material. The tungsten isotopic composition of this material shows more or less a terrestrial composition. However, the spinel-rich materials have excess 54Cr values (expressed as ε54Cr, which is the per ten thousand deviation of the 54Cr/52Cr ratio from a terrestrial standard) of around –0.4 to –0.5 ε54Cr, which distinctly point to an ordinary chondritic impactor. This result supports the asteroid impact interpretation but not the comet impact hypothesis.
Original languageEnglish
Title of host publication250 Million Years of Earth History in Central Italy
Subtitle of host publicationCelebrating 25 Years of the Geological Observatory of Coldigioco
Volume542
ISBN (Electronic)9780813795423
DOIs
Publication statusPublished - 2019

Publication series

SeriesGeological Society of America. Special Papers
ISSN0072-1077

Austrian Fields of Science 2012

  • 105105 Geochemistry

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